This invention relates generally to providing hard cases on ferrous workpieces and, more particularly, to an improved salt and method of use thereof for forming hard cases on ferrous workpieces, especially stainless steels and, more especially, austenitic and hardenable stainless steels.
It is known that ferritic nitrocarburizing at typical processing temperatures in various media, such as salt bath nitrocarburizing (also called salt bath nitriding) at about 1075xc2x0 F., enhances the corrosion resistance of carbon steels and low alloy steels, particularly if a post nitrocarburizing treatment, such as a quench-polish-quench, is employed. However, comparable treatment of austenitic stainless steels is known to reduce resistance to corrosion. Also, it is known that significant improvement in corrosion resistance over the as-processed corrosion resistance for nitrocarburized 420 grade and other hardenable stainless steel can be achieved by increasing the depth of the case hardened or compound layer on the surface thereof.
However, with the same treatment of austenitic stainless steels at 1075xc2x0 F. to increase the compound layer, a degradation of the corrosion resistance results. In fact, increasing the temperature of the salt bath to about 1166xc2x0 F. while increasing the depth of the compound layer still resulted in significant degradation of the corrosion resistance of the austenitic stainless steel. Thus, if one were to utilize conventional salt bath nitriding at conventional temperatures of 1075xc2x0 F. to achieve a compound layer of increased hardness to thereby increase wear resistance, one would be confronted with a resultant significant loss of corrosion resistance which is associated with and normally expected from austenitic grades of stainless steel.
Moreover, even though a significant depth of the compound layer formed on the surface of hardenable stainless steel, such as the 400 series, results in increased corrosion resistance, if this is achieved at a temperature above the tempering temperature, a reduction in core hardness will result; and 1075xc2x0 F., a conventional salt bath nitriding temperature, is above the desired tempering temperature of some products.
According to the present invention, a composition for nitrocarburizing stainless steel parts and a method for producing a nitride or hard case on such parts using the composition, sometimes referred to as salt bath nitriding, are provided. The composition includes alkali metal cyanate and alkali metal carbonate, wherein the cyanate ion (CNOxe2x80x94) is present in a weight percentage of greater than 45% and less than 53%. The composition is fused and maintained at a temperature of between about 750xc2x0 F. and about 950xc2x0 F. depending upon the type of stainless steel to be treated. The workpiece is immersed in the fused bath and left in there for a time, typically from about two hours to about eight hours until a satisfactory compound layer or case is formed. The best results are achieved with austenitic stainless steel wherein the piece is immersed from about three hours to about eight hours at temperatures between about 750xc2x0 F. and about 850xc2x0 F. With temperatures above 850xc2x0 F. to 950xc2x0 F., corrosion resistance is significantly reduced.
With respect to the hardenable 400 series stainless steel, increased corrosion resistance can be achieved by immersion for between four and six hours at 950xc2x0 F.